1. Field of the Invention
The present invention relates to a magnetic fluid bearing motor, and more particularly to a magnetic fluid bearing motor in which a long service life and cost reduction are achieved by employing a sleeve made of a porous material.
2. Detailed Description of the Related Art
Recently, bearings made of a porous material have been coming into wider and wider use in motors for recording apparatuses such as magnetic disk units or optical disk units, cooling fan units, and the like. This is because a porous bearing is excellent in moldability and thereby cost reduction is achieved with ease. Moreover, a technique has been developed that allows reduction in the hole rate of the surface of a bearing portion by forming a groove for generating dynamic pressure during molding. Such a technique started being applied to dynamic-pressure bearing motors.
However, a porous bearing has the following disadvantages. At a high-speed rotation, oil seeps out of the high-oil-pressure side of the bearing portion into the porous sleeve, but the flow rate of the oil flowing back to the low-oil-pressure side thereof is insufficient. This tends to cause running short of oil. Furthermore, if the loss of oil occurs in the bearing portion, in the oil carrying mechanism based on surface tension or diffusion process, the oil is diluted with air bubbles and thus the viscosity of the oil is substantially lowered. This causes serious problems in the dynamic-pressure generating function at a relatively early stage of operation.
To attain a desired dynamic-pressure generating function by using a combination of a porous sleeve and a dynamic-pressure bearing, it is necessary to achieve, with a simple construction, such as means for securing a flow passage through which the oil penetrated from the high-oil-pressure side of the bearing portion flows back to the low-oil-pressure side thereof without being scattered, and means for concentrating the oil held in the porous sleeve on the bearing portion and letting air bubbles remain around the outer circumference in a case of the loss of oil occurring in the bearing portion.
Although some attempts were made in the past to retain oil in the bearing portion reliably by using magnetic fluid oil as lubricating oil and using a magnet or the like, any of those attempts yielded no satisfactory results. For example, U.S. Pat. No. 3,746,407 proposes that a dynamic-pressure bearing portion and magnetic fluid oil are arranged inside a magnetic circuit of a magnet. Furthermore, Japanese Patent Laid-Open Publication No. Hei. 6-341434 proposes that a bearing portion, a porous material, magnetic fluid oil, and others are arranged inside a magnetic circuit of a magnet to retain the oil in the bearing portion. Japanese Utility Model Laid-Open Publication No. Hei. 3-49416 proposes that a porous sleeve formed of a ferromagnetic substance is magnetized to retain magnetic fluid oil inside the sleeve.
However, oil-retaining mechanisms as proposed in the above-mentioned patent applications aim at simply retaining oil around a bearing portion and pay no regard to methods for securing a proper quantity of oil which circularly flows and for maintaining the viscosity of the oil held in the bearing portion by forming three-dimensional concentration gradient inside the porous sleeve in consideration of the existence of oil diluted with air bubbles. This causes a dynamic-pressure bearing to suffer from critical defects. Furthermore, a bearing structure in which a magnetic circuit is formed requires a large number of associated parts, such as a magnet, and is thus not suitable for use in low-cost products.
In view of the foregoing, an object of the present invention is to provide a magnetic fluid bearing motor that has a long service life and requires less cost in which concentration gradient of oil is formed by realizing magnetic field gradient with a simple construction inside the porous sleeve.
According to the present invention, a magnetic fluid bearing motor is provided with a bearing assembly composed of a shaft, a substantially solid porous sleeve moving in rotation relatively to the shaft with a minute gap provided therebetween, and magnetic fluid oil, wherein one or more bearing surfaces of the sleeve corresponding to a radial bearing, a thrust bearing, and the like, or the shaft is magnetized to create magnetic flux density gradient inside the porous sleeve, the magnetic flux density gradient being set at the maximum on the bearing surface.
More specifically, a magnetization pattern is created on the shaft formed of a ferromagnetic substance such that the magnetization-varying portion is so located as to correspond to the bearing portion. Otherwise a magnetization pattern is created on the porous sleeve formed of a ferromagnetic substance such that the magnetization-varying portion is so located as to correspond to the surface of the bearing portion.
Moreover, a magnetization pattern to be created on the bearing surface of the sleeve or the shaft is so controlled that, in the spiral- or herringbone-shaped dynamic-pressure generating groove, the magnetization-varying portion is so located as to correspond to the oil-admission end thereof.
Since the leakage of the magnetic flux from the magnetization is diffused, three-dimensional magnetic flux density gradient is formed inside the porous sleeve. The farther the magnetic flux density gradient lies away from the sleeve surface of the bearing portion, the smaller it becomes. This means that, since a magnetic substance is generally driven to be attracted toward a direction in which magnetic flux density is made high, a field of force is created for driving the magnetic fluid oil to be attracted toward the bearing portion.
Accordingly, since the porous sleeve has been impregnated with magnetic fluid oil, the less the quantity of the magnetic fluid oil due to evaporation, migration, or other reasons, the more the magnetic fluid oil is collected in the vicinity of the bearing portion of the sleeve and air bubbles remain in the outer peripheral portion thereof, i.e., the oil has dense and rarefied portions. This makes it possible to retain the magnetic fluid oil reliably in the bearing portion without it being diluted with air bubbles.
The porous sleeve is not so designed that only the bearing portion is made of a porous material but so designed that the entire structure thereof including unnecessary parts provided around the bearing portion is substantially made of a single porous material. Setting the volume of the porous sleeve at the maximum allows the range of magnetic fluid oil capacity to be made larger, and thereby the magnetic fluid oil can be divided into dense and rarefied portions in the radial or axial direction. This makes it possible to secure as large available magnetic fluid oil quantity as possible.